Dental materials which are designed to remain in the mouth of a patient are often radiopaque which enables the practitioner to identify them on an x-ray film. This is of particular importance if the dental material is highly aesthetic and thus cannot be easily distinguished from natural tooth structure.
Radiopacity is typically imparted to a dental material by incorporation of an inorganic filler containing heavy metal elements. Examples of radiopaque fillers used in dentistry include ground glasses containing heavy metal elements such as strontium or barium, various metal oxide nanoparticles (e.g. zirconia also referred to as nano-zirconia) which can either be used as discrete particles, or as clusters of particles together with, e.g. silica particles, and insoluble salts of heavier rare earths, such as yttrium fluoride or ytterbium fluoride.
For several reasons, however, it is often desirable to have small-sized radiopaque additives (e.g., on the nanometer scale) for use in dental materials:                for more highly viscous materials such as filling composites and luting cements, it is desirable to have small additives to fill in the gaps between larger filler particles in order to achieve high filler loading and high mechanical strength;        for lower viscous materials, such as adhesives or sealants, small additives are desired to prevent settling of the filler out of the liquid, and thus maintain a homogeneous material throughout its shelf life; and        for all aesthetic dental materials, small additives with a size below the wavelength of visible light are desired, because they do not scatter or reflect visible light and thus enable translucent materials.E.g. WO 2012/64573 A1 (Shukla et al.) describes a radiopaque dental adhesive composition comprising radiopaque metal oxide nanoparticles (e.g. zirconia).        
US 2003/0004294 (Moszner et al.) relates to dental materials with improved mechanical properties containing polymerizable metal oxide clusters of a particular formula. It is described that the curing of these materials can be accomplished by thermal, photochemical or redox-induced polymerization.
Incorporating zirconia nanoparticles in dental compositions, however, is not always easy as these particles are sometimes incompatible with (meth)acrylate functionalized polyalkenoic acids, especially in the presence of water. Upon mixing sometimes a kind of gellation process occurs which is not desired especially from a shelf life point of view.